A2 Biology: Sexual Reproduction in Plants
Revision notes on sexual reproduction in plants:
-Structure, cross/self-pollination, fertilisation, germination
- Created by: Pennaling
- Created on: 03-05-14 22:43
Intro
Angiosperms:- group of plants that produce flowers that contain sex organs.
Many common flowers contain both male and female sex organs = HERMAPHRODITE
Male gametes --> enclosed IN a pollen grain
Female gametes --> enclosed IN an ovule In an ovary
Structure of an Insect-pollinated Flower
Structure of an Insect-pollinated Flower
All Sepals = CALYX --> protects young flower bud
All Petals = COROLLA --> attracts pollinators with colour + shape
Receptacle = part of the stalk where other parts attach
Anther = produces pollen containing male gametes
Filament = supports anther
Stigma = receptive surface for pollen grains (sticky)
Style = joins stigma to ovary + pollen tube goes through it
Ovary = contains ovule + becomes the fruit
Ovule = carries female gametes + becomes seed on fertilisation
Pollination
POLLINATION = the movement of the pollen grain from the anther to the stigma of a plant of the same species
- Self-pollination:- pollen on the stigma comes from a flower on the same plant or even the same flower
- Reduced variation
- Both gametes produced from meiosis of the same genome
- Preserves genomes suited to environment
- More likely to bring together unfavourable recessive alleles
- Self-pollination better than nothing as it will produce more variation than asexual reproduction
- Cross-pollination:- pollen on the stigma comes from a flower on a different plant of the same species
- Meiosis in both plants produce haploid gametes
- Greater variation
- Through evolution - significant as some individuals will be more successful = NATURAL SELECTION
Mechanisms to ensure cross-pollination:-
- Anthers + stigmas mature at different times (protandry + protogyny), anther + stigma at different levels
Insect-pollinated Flowers
Usually large, brightly coloured + scented to attract insects.
- Nectar is often produced to attach insects
- Pollen grains are large, sticky + heavy, usually with rough surfaces so that they can stick to the insects bodies
- Petals often enclose the stamens + carpels so insects rub against them when feeding
- Stigmas are sticky so that pollen grains setting on them are not easily displaced
Wind-pollinated Flowers
Often small + dull since they don't need to attract insects. No nectar or scent.
- Lots of pollen produced as there is a lot wasted, never reaching a stigma
- Pollen grains are small, dry, smooth + light, so that they are buoyant + easily blown about by air currents
- Stamens usually have long, slender filaments that hang outside the flower to catch the wind, shaking out the pollen from the anther
- Stigmas protrude + are large + feathery so that they provide a large surface area to catch pollen gloating in the air
Male Gamete
Enclosed INSIDE a pollen grain
Each anther contains 4 pollen sacs - many pollen grains in each sac
DEHISCENCE = splitting open of the anther to release the pollen grains
Female Gamete
Enclosed INSIDE ovule
Megasporocyte or megaspore mother cell divides by meiosis to form 4 haploid cells
- 3 degenerate + the remaining one forms and embryo sac
- The haploid cell within the embryo sac divides by meiosis 3 times --> producing 8 haploid nuclei
Fertilisation
The fusion of male (n) + female (n) gametes to produce a zygote (2n)
- Male gametes IN pollen grain, female gametes IN ovary
Male gametes can ONLY reach the female gametes by means of a POLLEN TUBE
Pollen Tube Growth
The male gamete must reach the female gamete.
- Pollen grain lands on stigma - it absorbs water, swells + splits open = germination of pollen grain --> producing pollen tube
- Pollen tube grows from the pollen grain, down the style (tube growth controlled by pollen tube nucleus)
- Tube secretes enzymes which digest a pathway through the style
- Once near the ovary the generative nucleus divides by mitosis --> forming 2 haploid nuclei
- Pollen tube enters the ovule through the micropyle (the end of the tube and tube nucleus degenerate)
- The tip of the pollen tube bursts open releasing the male gamete into the embryo sac + the two male nuclei enter
- One of the male gametes fuses with the female egg nucleus to form the zygote, which will divide to form an embryo
- The other male nucleus fuses with both polar bodies --> forming triploid endosperm nucleus (3n) - food store for developing embryo
- Zygote becomes the embryo plant
Seed Germination
Zygote --> embryo plant: consisting of
- plumule - the future shoot
- radicle - the future root
Integument --> testa (seed coat)
Micropyle --> remains where it is; allowing water to enter the seed during germination
Ovary --> becomes the fruit - a ripened ovary containing the seeds e.g. squash, tomatoes, beans, corn, grains
Cotyledons (seed leaf)
- Monocotyledons --> only one cotyledon, food store surrounds the cotyledons e.g. cereal grains
- Dicotyledons --> two cotyledons, food store absorbed into the cotyledons (contains endosperm) e.g. broad bean
Seed Germination
Broad bean germination, requires:
- Suitable temperature --> optimum temperature required for enzymes, required for germination (spring)
- Water --> helps to transport material around the seedling e.g. nutrients
- Oxygen --> Respiration - ATP energy for growth of the seedling
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